This invention relates to a spark plug having noble metallic or comparable tips fixed to opposed center and ground electrodes so as to cause spark discharge between them. Furthermore, this invention relates to a method for manufacturing this spark plug.
This kind of spark plug is for example disclosed in the unexamined Japanese patent publication No. 52-36237, as schematically shown in FIGS. 19A and 19B. First, an arrangement shown in FIG. 19A comprises a noble metallic tip (i.e., center electrode tip) J2 fixed to an apical end of a center electrode J1 and a noble metallic tip (i.e., ground electrode tip) J4 fixed to an apical end of a ground electrode J3. The center electrode tip J2 and the ground electrode tip J4 are opposed to each other along an axis of the center electrode J1.
Second, an arrangement shown in FIG. 19B comprises the center electrode tip J2 fixed to an apical end of the center electrode J1 and the ground electrode tip J4 fixed to an apical end of the ground electrode J3. The center electrode tip J2 and the ground electrode tip J4 are opposed to each other along a line normal to the axis of the center electrode J1.
However, according to the arrangement shown in FIG. 19A, the ground electrode J3 is long and therefore the heat releasing ability of ground electrode J3 is insufficient. When subjected to the heat of combustion, the ground electrode J3 will have a high temperature. This worsens the heat resistance and lowers the mechanical strength of the ground electrode J3. Furthermore, the noble metallic tip of the ground electrode will be worn hardly. Accordingly, it is difficult to assure a proper life of the ground electrode.
In general, the flow of gas mixture in a combustion chamber is normal to the axis of a plug (i.e., the axis of center electrode J1) as shown by an arrow Y in FIG. 19B. Such a flow of gas mixture tends to forcibly shift a flame kernel toward the ground electrode J3 when the flame kernel is produced in the discharge gap between the opposed tips J2 and L4 in response to an ignition.
In this case, according to the arrangement of FIG. 19B, the center electrode J1 and the ground electrode J3 are positioned close and parallel to each other. Hence, due to the flow of gas mixture, the flame kernel collides with the ground electrode J3 and is cooled by the ground electrode J3. This worsens the ignitability of a spark plug.
Furthermore, this kind of spark plug is disclosed in the unexamined Japanese patent publication No. 61-45583, as schematically shown in FIG. 20. An arrangement shown in FIG. 20 comprises a ground electrode J6 having a proximal portion fixed to a metallic housing J5 and a distal portion extending toward an apex of center electrode J7 so that an acute angle is formed between an axis of the distal portion of ground electrode J6 and the axis of center electrode J7.
The arrangement shown in FIG. 20 is advantageous in that the ground electrode is short in length and excellent in heat resistance and mechanical strength when compared with an ordinary ground electrode having a distal portion perpendicular to an axis of the center electrode and overhanging an apex of the center electrode (refer to FIG. 19A).
However, according to the arrangement shown in FIG. 20, a ground electrode tip J8 is provided within an area of an end surface J61 of the distal portion of ground electrode J6. A distance J10 between the distal portion of ground electrode J6 and the center electrode tip J9 must be short to form an appropriate discharge gap between the ground electrode tip J8 and the center electrode tip J9.
Accordingly, when the flame kernel is shifted toward the ground electrode J6 due to the flow of gas mixture, the flame kernel is cooled by the ground electrode J6. The ignitability of a spark plug becomes dissatisfactory. Furthermore, as a fixing portion of the ground electrode tip J8 to the ground electrode J6 is positioned close to the center electrode tip J9, a discharge may occur at the fixing portion of the ground electrode tip J8. Thus, the reliability of the tip fixing portion cannot be assured.
In view of the foregoing problems of the prior art, the present invention has an object to provide a spark plug having noble metallic or comparable tips fixed to opposed center and ground electrodes so as to cause spark discharge between them which is capable of shortening the length of the ground electrode and improving the heat resistance and mechanical strength, and also capable of preventing the discharge from occurring from a tip fixing portion of the ground electrode, thereby assuring the fixing reliability of the tip and realizing excellent ignitability.
To accomplish the above and other related objects, the present invention provides a first spark plug comprising a cylindrical metallic housing, a center electrode accommodated in the metallic housing with one end protruding and extending from one end of the metallic housing, a center electrode tip fixed to the one end of the center electrode and extending in the same direction as an axis of the center electrode, a ground electrode having a proximal portion fixed to the one end of the metallic housing and a distal portion extending toward the one end of the center electrode, and a columnar ground electrode tip fixed to an end surface of the distal portion of the ground electrode with an apical surface of the ground electrode tip opposed to an apical surface of the center electrode tip via a discharge gap. The first spark plug is characterized in that an acute angle is formed between an axis of the distal portion of the ground electrode and the axis of the center electrode when the ground electrode is projected on a virtual plane including the axis of the center electrode and a cross-sectional centroid of a proximal end of the ground electrode where the ground electrode is fixed to the metallic housing. The ground electrode tip extends along an axis crossing with the axis of the distal portion of the ground electrode, so that the ground electrode tip protrudes from the end surface of the distal portion of the ground electrode and extends toward the center electrode. And, an axis of the center electrode tip is in a cross or skew relationship with an axis of the ground electrode tip.
According to the first spark plug, the ground electrode having the proximal portion fixed to one end of the metallic housing has the distal portion extending toward one end of the center electrode so that an acute angle is formed between the axis of the distal portion of the ground electrode and the axis of the center electrode. Thus, it becomes possible to shorten the length of the ground electrode and improve the heat resistance and mechanical strength when compared with an ordinary ground electrode having a distal portion perpendicular to the axis of the center electrode and overhanging an apex of the center electrode.
Furthermore, according to the first spark plug, the columnar ground electrode tip protrudes from the end surface of the distal portion of the ground electrode and extends toward the center electrode in the direction crossing with the axis of the distal portion of the ground electrode. And, the axis of the center electrode tip is in a cross or skew relationship with the axis of the ground electrode tip. Thus, it becomes possible to provide an appropriate distance between the fixing portion of the ground electrode tip and the apical surface of the center electrode tip which is sufficiently longer than a distance between the apical surface of the ground electrode tip and the apical surface of the center electrode tip.
Namely, the distance from the apical surface of the ground electrode tip to the apical surface of the center electrode tip is sufficiently shorter than the distance from the fixing portion of the ground electrode tip to the apical surface of the center electrode tip. This ensures that the discharge only occurs between the apical surfaces of the opposed electrode tips, and therefore prevents the discharge from occurring from the fixing portion of the ground electrode tip.
Furthermore, according to the first spark plug, a distance from the distal portion of the ground electrode to the apical surface of the center electrode tip is so long as not to obstruct the growth of flame kernel caused between the apical surfaces of the opposed electrode tips.
Accordingly, the first spark plug of the present invention is capable of shortening the length of the ground electrode and improving the heat resistance and mechanical strength, and also capable of preventing the discharge from occurring from the fixing portion of the ground electrode tip, thereby assuring the fixing reliability of the tip and realizing excellent ignitability.
According to the first spark plug, it is preferable that a crossing angle between the axis of the center electrode tip and the axis of the ground electrode tip is in an angular range from 5xc2x0 to 70xc2x0.
If the crossing angle is less than 5xc2x0, the ground electrode will have substantially the same configuration as that of a conventional one which overhang the center electrode. The heat resistance and mechanical strength will be worsened. On the other hand, if the crossing angle is larger than 70xc2x0, the distal portion the ground electrode will be positioned so close to the center electrode tip that the growth of flame kernel is obstructed by the distal portion the ground electrode and therefore the ignitability is worsened.
Furthermore, the present invention provides a second spark plug comprising a cylindrical metallic housing, a center electrode accommodated in the metallic housing with one end protruding and extending from one end of the metallic housing, a center electrode tip fixed to the one end of the center electrode and extending in the same direction as an axis of the center electrode, a ground electrode having a proximal portion fixed to the one end of the metallic housing and a distal portion extending toward the one end of the center electrode so that an acute angle is formed between an axis of the distal portion of the ground electrode and the axis of the center electrode, and a columnar ground electrode tip fixed to an end surface of the distal portion of the ground electrode or fixed to a side surface of the distal portion of the ground electrode facing to the center electrode with an apical surface of the ground electrode tip opposed to an apical surface of the center electrode tip via a discharge gap. The second spark plug is characterized in that the ground electrode tip extends along an axis crossing with the axis of the distal portion of the ground electrode, so that the ground electrode tip protrudes from the end surface of the distal portion of the ground electrode and extends toward the center electrode. A crossing angle between an axis of the center electrode tip and an axis of the ground electrode tip is in an angular range from 5xc2x0 to 70xc2x0. And, a fixing portion of the ground electrode tip to the ground electrode is far from the metallic housing in an axial direction of the center electrode compared with the apical surface of the center electrode tip.
According to the second spark plug, it is preferable that the ground electrode tip protrudes toward the center electrode by a protruding length in a range from 0.3 mm to 1.5 mm with respect to the side surface of the distal portion of the ground electrode.
If the length is less than 0.3 mm, the distal portion of the ground electrode will be located so close to the center electrode tip that the growth of flame kernel is obstructed by the distal portion the ground electrode. On the other hand, if the length is larger than 1.5 mm, the ground electrode tip becomes so long that heat release becomes insufficient and the durability against oxidizing exhaustion becomes weak.
Furthermore, the present invention provides a third spark plug comprising a cylindrical metallic housing, a center electrode accommodated in the metallic housing with one end protruding and extending from one end of the metallic housing, a center electrode tip fixed to the one end of the center electrode and extending in the same direction as an axis of the center electrode, a ground electrode having a proximal portion fixed to the one end of the metallic housing and a distal portion extending toward the one end of the center electrode, and a columnar ground electrode tip fixed to a side surface of the distal portion of the ground electrode facing to the center electrode with an apical surface of the ground electrode tip opposed to an apical surface of the center electrode tip via a discharge gap. The third spark plug is characterized in that an acute angle is formed between an axis of the distal portion of the ground electrode and the axis of the center electrode when the ground electrode is projected on a virtual plane including the axis of the center electrode and a cross-sectional centroid of a proximal end of the ground electrode where the ground electrode is fixed to the metallic housing. And, an axis of the center electrode tip is in a cross or skew relationship with an axis of the ground electrode tip.
The third spark plug is characterized in that the ground electrode tip is fixed to the side surface of the distal portion of the ground electrode facing to the center electrode, and therefore differs from the first spark plug which has the ground electrode tip fixed to the end surface of the distal portion of the ground electrode. According to the third spark plug, the ground electrode tip necessarily protrudes toward the center electrode tip than the fixing portion of the ground electrode tip.
Thus, like the first spark plug, the third spark plug can shorten the length of the ground electrode and improve the heat resistance and mechanical strength.
Furthermore, not only the ground electrode tip is fixed to the side surface of the distal portion of the ground electrode facing to the center electrode but also the axis of the center electrode tip is in a cross or skew relationship with the axis of the ground electrode tip. Thus, it becomes possible to provide an adequate distance between the fixing portion of the ground electrode tip and the apical surface of the center electrode tip which is sufficiently longer than a distance between the apical surface of the ground electrode tip and the apical surface of the center electrode tip.
Hence, like the first spark plug, the third spark plug ensures that the discharge only occurs between the apical surfaces of the opposed electrode tips, and therefore prevents the discharge from occurring from the fixing portion of the ground electrode tip. Furthermore, a distance from the distal portion of the ground electrode to the apical surface of the center electrode tip is so long as not to obstruct the growth of flame kernel caused between the apical surfaces of the opposed electrode tips.
Accordingly, the third spark plug of the present invention is capable of shortening the length of the ground electrode and improving the heat resistance and mechanical strength, and also capable of preventing the discharge from occurring from the fixing portion of the ground electrode tip, thereby assuring the fixing reliability of the tip and realizing excellent ignitability.
Even in the third spark plug, it is preferable that a crossing angle between the axis of the center electrode tip and the axis of the ground electrode tip is in an angular range from 5xc2x0 to 70xc2x0. Furthermore, it is preferable that the ground electrode tip protrudes toward the center electrode by a protruding length in a range from 0.3 mm to 1.5 mm with respect to the side surface of the distal portion of the ground electrode.
Furthermore, the present invention provides a fourth spark plug comprising a cylindrical metallic housing, a center electrode accommodated in the metallic housing with one end protruding and extending from one end of the metallic housing, a center electrode tip fixed to the one end of the center electrode and extending outward from the center electrode, a ground electrode having a proximal portion fixed to the one end of the metallic housing and a distal portion extending toward the one end of the center electrode, and a columnar ground electrode tip fixed to an end surface of the distal portion of the ground electrode with an apical surface of the ground electrode tip opposed to an apical surface of the center electrode tip via a discharge gap. The fourth spark plug is characterized in that an acute angle is formed between an axis of the distal portion of the ground electrode and the axis of the center electrode when the ground electrode is projected on a virtual plane including the axis of the center electrode and a cross-sectional centroid of a proximal end of the ground electrode where the ground electrode is fixed to the metallic housing. The ground electrode tip extends along an axis crossing with the axis of the distal portion of the ground electrode, so that the ground electrode tip protrudes from the end surface of the distal portion of the ground electrode and extends toward the center electrode. And, the axis of the center electrode is in a cross or skew relationship with an axis of the ground electrode tip.
The fourth spark plug is characterized in that the axis of the center electrode tip is not specifically defined with respect to the axis of the center electrode, and therefore differs from the first spark plug which has the axis of the center electrode tip extending in the same direction as the axis of the center electrode. Furthermore, according to the fourth spark plug, the axis of the ground electrode tip is in a cross or skew relationship with the axis of the center electrode.
Thus, like the first spark plug, the fourth spark plug can shorten the length of the ground electrode and improve the heat resistance and mechanical strength.
Furthermore, according to the fourth spark plug, the columnar ground electrode tip protrudes from the end surface of the distal portion of the ground electrode and extends toward the center electrode in the direction crossing with the axis of the distal portion of the ground electrode. And, the axis of the center electrode is in a cross or skew relationship with the axis of the ground electrode tip. Thus, it becomes possible to provide an appropriate distance between the fixing portion of the ground electrode tip and the apical surface of the center electrode tip which is sufficiently longer than the distance between the apical surface of the ground electrode tip and the apical surface of the center electrode tip.
Hence, like the first spark plug, no discharge occurs from the fixing portion of the ground electrode tip and the ground electrode does not obstruct the growth of flame kernel.
Accordingly, the fourth spark plug of the present invention is capable of shortening the length of the ground electrode and improving the heat resistance and mechanical strength, and also capable of preventing the discharge from occurring from the fixing portion of the ground electrode tip, thereby assuring the fixing reliability of the tip and realizing excellent ignitability.
Furthermore, the present invention provides a fifth spark plug comprising a cylindrical metallic housing, a center electrode accommodated in the metallic housing with one end protruding and extending from one end of the metallic housing, a center electrode tip fixed to the one end of the center electrode and extending outward from the center electrode, a ground electrode having a proximal portion fixed to the one end of the metallic housing and a distal portion extending toward the one end of the center electrode, and a columnar ground electrode tip fixed to a side surface of the distal portion of the ground electrode facing to the center electrode with an apical surface of the ground electrode tip opposed to an apical surface of the center electrode tip via a discharge gap. The fifth spark plug is characterized in that an acute angle is formed between an axis of the distal portion of the ground electrode and the axis of the center electrode when the ground electrode is projected on a virtual plane including the axis of the center electrode and a cross-sectional centroid of a proximal end of the ground electrode where the ground electrode is fixed to the metallic housing. And, the axis of the center electrode is in a cross or skew relationship with an axis of the ground electrode tip.
The fifth spark plug is characterized in that the ground electrode tip is fixed to the side surface of the distal portion of the ground electrode facing to the center electrode, and therefore differs from the fourth spark plug which has the ground electrode tip fixed to the end surface of the distal portion of the ground electrode. According to the fifth spark plug, the ground electrode tip necessarily protrudes toward the center electrode tip than the fixing portion of the ground electrode tip.
Accordingly, from the same reason described above, the fifth spark plug of the present invention is capable of shortening the length of the ground electrode and improving the heat resistance and mechanical strength, and also capable of preventing the discharge from occurring from the fixing portion of the ground electrode tip, thereby assuring the fixing reliability of the tip and realizing excellent ignitability.
Even in the fourth or fifth spark plug, it is preferable that a crossing angle between the axis of the center electrode and the axis of the ground electrode tip is in an angular range from 5xc2x0 to 70xc2x0. Furthermore, it is preferable that the ground electrode tip protrudes toward the center electrode by a protruding length in a range from 0.3 mm to 1.5 mm with respect to the side surface of the distal portion of the ground electrode.
When the axis of the center electrode is in a cross or skew relationship with the axis of the ground electrode tip, spark discharge causes non-uniform exhaustion (i.e., uneven wear) on the apical surface of each tip. This will enlarge the discharge gap and shorten the life of a plug.
For example, simply increasing the diameter of a tip (i.e., using a thick tip) will assure a practical level of plug life (e.g., equivalent to 100,000 Km in terms of vehicle traveling distance). However, a thick tip will obstruct the growth of flame kernel during the discharge and accordingly sacrifice the ignitability.
In view of this problem, the inventors of the present invention have conducted experiments to optimize the relationship between opposed electrode tips for assuring a practical level of plug life and for obtaining reliable wear durability.
According to the result of such experiments, in any of the first to third spark plugs, it is preferable that an X axis represents the apical surface of the center electrode tip and a Y axis represents the axis of the center electrode tip in a coordinate plane including both of the axis of the center electrode tip and the axis of the ground electrode tip, with a crossing point of the X axis and the Y axis being an origin (0, 0) of the coordinate plane. And, a point xe2x80x98Axe2x80x99 of the ground electrode tip closest to the center electrode tip is expressed by a coordinate value (xe2x88x92b/2, "khgr") when a point xe2x80x98Bxe2x80x99 on the apical surface of the center electrode tip closest to the ground electrode is expressed by a coordinate value (xe2x88x92b, 0), where "khgr" represents a discharge gap. Furthermore, an axial deviation amount between the axis of the center electrode tip and the axis of the ground electrode tip is within xc2x1d/2 in a direction normal to the coordinate plane, and a swing amount of the closest point xe2x80x98Axe2x80x99 is within xc2x1d/2 in a direction parallel to the X axis, wherein xe2x80x98dxe2x80x99 represents a diameter of the ground electrode tip.
This arrangement brings the effect of assuring a practical level of plug life by suppression the wear of the center and ground electrode tips in addition to the effects brought by the first to third spark plugs.
When the discharge gap is 1.05 mm, an enlargement of the discharge gap due to the wear of tips must be less than or equal to 1.4 mm. The spark plug according to the above optimization can suppress such an enlargement of the discharge gap to a value within 1.4 mm during a practical level of plug life.
The above optimization can be also utilized in manufacturing the spark plug of the present invention.
In this case, it is desirable that the center electrode tip has a cylindrical shape with a cross section in a range from 0.07 mm2 to 0.79 mm2, and the ground electrode tip has a cylindrical shape with a cross section in a range from 0.07 mm2 to 1.13 mm2.
If the diameter (the cross section) of each tip is excessively large, the flame kernel will collide the tip. In other words, the growth of flame kernel will be obstructed by the tip. On the other hand, if the diameter (the cross section) of each tip is excessively small, heat release from the tip will be worsened. Exhaustion of tip will be promoted. The above-defined range of the tip diameter is the result optimized through the study of influence given to the ignitability and heat resistance of a tip.
Furthermore, it is preferable that the ground electrode has a tapered shape with a cross-sectional area gradually narrowing with decreasing distance from the end surface. This arrangement effectively reduces an area of the ground electrode contacting with the flame kernel. Accordingly, the ignitability can be improved.
Furthermore, it is preferable that the ground electrode has an outer layer made of a Ni alloy and an inner layer made of a copper or copper alloy. Due to excellent thermal conductivity of a copper or copper alloy, this arrangement effectively improves the heat releasing ability of the ground electrode.
Furthermore, it is preferable that the center electrode tip and said ground electrode tip are made of a Pt alloy including at least one additive selected from the group consisting of Ir, Ni, Rh, W, Pd, Ru and Os. More specifically, a preferable material for the center electrode tip and the ground electrode tip is a Pt alloy containing at least one additive selected from the group consisting of Ir (50 weight % or less), Ni (40 weight % or less), Rh (50 weight % or less), W (30 weight % or less), Pd (40 weight % or less), Ru (30 weight % or less), and Os (20 weight % or less).
Furthermore, it is preferable that the center electrode tip and the ground electrode tip are made of a Ir alloy including at least one additive selected from the group consisting of Rh, Pt, Ni, W, Pd, Ru and Os. More specifically, a preferable material for the center electrode tip and the ground electrode tip is a Ir alloy containing at least one additive selected from the group consisting of Rh (50 weight % or less), Pt (50 weight % or less), Ni (40 weight % or less), W (30 weight % or less), Pd (40 weight % or less), Ru (30 weight % or less), and Os (20 weight % or less).